What is a catalyst?

• A. A substance that slows down a reaction.
• B. A substance that speeds up a reaction without being consumed.
• C. A substance that is consumed in a reaction.
• D. A substance that changes the color of the reaction.

Answer: (B)

A catalyst is a substance that speeds up a chemical reaction by providing an alternative pathway with lower activation energy, and it is not consumed in the process. Because it isn’t used up, it can keep helping more molecules react, making the reaction happen faster without changing the final amounts of reactants and products (though it can affect how quickly equilibrium is reached). Examples include enzymes acting as biological catalysts and platinum in catalytic converters. The other ideas describe an inhibitor (which slows reactions), a reactant that is consumed, or something that merely changes color—none of which define a catalyst.